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Design Basics

The factors that affect sizing and performance of a collector are the material (dust) itself, the temperature effect on the air, gas, product, fineness of the material (fume being an example), dust, and particulate loading in grains per cubic foot. These factors determine the type of collector selected, the housing construction required, inlet locations, fabric medium selection, and dust discharge parameters. Dust collector manufacturers distribute application data inquiry forms that provide the answers to questions needed to specify the correct collector design and arrangement for a given application. For example, it is important to know if the dust is explosive, statically charged, hygroscopic light, heavy, fine, wet, sticky, and so on. Do we need insulation, hopper heaters, and special equipment for discharging dust? Is the collector located inside or outside? Does the exhaust air go back to plant or outside? These are just a few serious questions meant to indicate just how important it is for us to know the details before specifying any collector.

After analyzing these parameters, the designer can then choose from among a wide variety of fabric filter collectors to solve the emissions problem. The most basic type is the shaker collector, named after its use of a shaking mechanism to dislodge accumulated particulate.

The shaker collector has tubular socks of a woven medium suspended by a strap on the top of the bag connected to a mechanical shaking arm. No cages are required to hold the bags open, and the lower end of the bag socks are clamped to the tube sheet located directly above the hopper. The dirty air enters the unit in the hopper section and is forced to go upward inside the socks. When the socks get plugged (blinded), the differential pressure goes up. This creates an electrical signal that shuts off the fan or closes a damper and shuts off the air flow into the collector. The shaker mechanism then shakes the filter socks for an adjustable period, dislodging the dust cake and allowing it to fall back down into the hopper. Shakers use a light woven fabric medium designed to be very flexible. After a time, the shaking stops, the damper opens, and air flows through the collector. The problem with the shaker is that it cannot operate continuously because the process air and ventilation system must be shut down for it to clean. To achieve continuous operation, compartmentalized shaker units with some modules operating on-line cleaning process air and with some modules off-line cleaning filters are required. Also, the light-woven, flexible filter medium is not particularly efficient at removing the dust from the air, making the shaker suspect as an air pollution control device. The shaker is considered a low-energy intermittent use collector. The filter medium does not get worked very forcefully during cleaning, which can be an asset relative to filter life in high heat or corrosive applications.

The reverse air collector is built in numerous configurations. It is a moderate energy device. Generally, it uses a caged needled fabric tubular medium, making it a rather good choice for air pollution control applications. The reverse air cleaning principle is to use an extra air mover for cleaning filters. This extra air mover produces a higher pressure than the air flowing through the collector; hence, a flow of air through the cage and medium from the clean side of the filter dislodges the particulate from the dirty side, allowing it to fall into the hopper. The frequency and duration of the cleaning cycle is much the same as the shaker type. This reverse air flow is usually better at cleaning than gently shaking the filter bag. The time of the cleaning cycle is much the same as the shaker. Again, this is particularly true when the collector is setup in modular fashion with some sections of the collector online cleaning process air and some sections off-line cleaning filters. Cleaning the filters off-line is easy because there is no process air pressure holding particulate on the filter bag surface. The only real problem with reverse air collection is that controlling the air, on and off, during cleaning cycles on modular arrangements is complex and costly. Figure 7.5 shows an industrial reverse air collector. The moving arm in the center of the vessel applies a reverse pulse of air to individual tube rows. Other reverse air collectors break the housing into compartments using isolation valves. Using blowers, the air flow through the compartment being cleaned can be reversed, thereby cleaning the medium.

Some reverse air collectors are built with tube sheets low, directly above the hopper, with dirty air flowing upward inside uncaged bags, and with the tube sheet high, under the CAP, with dirty air flowing to the outside of caged bags. Reverse air collectors are also built in a tall cylindrical form configuration as in Figure 7.5. Typically, operating on-line, a continuously revolving arm blowing the higher cleaning air pressure down inside the filters is used, as in our previous example. The solid product falls between the bags into the hopper. The round unit with the single revolving cleaning arm is a single module, cleaning a few filters at a time on-line, making it a stand-alone collector. This is especially true when the reverse air cleaning fan is located within the collector. Some models require an external fan or blower for cleaning energy, which adds to complexity, cost, space, and moist air cleaning potentials.

An inherent problem with round collectors is that they do not use filter space well. Many more filters can be in a square or rectangular configuration. This becomes increasingly important in large installations in the

FIGURE 7.5

Reverse air collector (Donaldson Company, Inc.).

space-saving sense. Also, the tall form, cylindrical design does not lend itself to the architectural aesthetics of the modern low-profile industrial park. However, all in all, the reverse air does excel in some applications, especially grain, wood, paper, and other floating particulate. The cleaning cycle off-line is long enough to free the dust from the filter for an appreciable time so it can drop into the hopper. The model with a low tube sheet with uncaged filter bags is a good choice for heavy loadings in hot lime, cement, and kiln processing applications as the cleaning energy is not too intense to break filters down. Also, the mineral product is heavy enough to drop out of the bags and gravitate to the hopper.

The pulse jet collector is a high-energy cleaner as it uses high-pressure air blown down inside caged filter bags in bursts of 20-80 ms. The pulse jet uses filter bags with cages that are suspended from tube sheets between the DAP and CAP. Needled felt filters are used for high-cleaning-efficiency style, making it a good air pollution control device. This high-pressure air is typically directed through a Venturi to increase air volume, raises the air pressure inside the filter over the process air flowing through the collector, and the shock wave blasts the particulate off the filter bag, where it drops into the hopper. The pulse jet can be round, square, rectangular, short, tall, large, or small. It can be modified easily for trough, pyramid hoppers, high or low inlets, and walk-in or trapdoor CAPs, allowing for service in a clean-air atmosphere. It uses common factory-compressed air for cleaning instead of an extra fan or positive displacement blower. Some problems associated with pulse jets are that the high energy imparted to the filter breaks filter media down, particularly in high heat and/or chemical corrosive atmospheres. Also, the location of the Venturi is important with respect to the tube sheet. With the Venturi located in the filter bag itself, a negative air pressure exists above the Venturi lip down in the bag area, creating a suction pressure rather than positive air pressure at the top of the bag during cleaning. This leads to buildup of product under the tube sheet. It also takes the filter area of an 8-ft bag and effectively turns it to that of a 7-ft bag. A Venturi above the tube sheet eliminates this phenomenon.

The isometric view of a pulse jet collector is shown in Figure 7.6. In this unit, the gas inlet plenum is shown to the lower left and the cleaned gas

FIGURE 7.6

Pulse jet baghouse (Bionomic Industries, Inc.).

FIGURE 7.7

Pulse jet collector with high gas inlet (Steelcraft Corp.).

outlet is at the upper right, as part of a discharge plenum. The cutaway shows the bags arranged in rows in the collector. The bag access is through the top of this design. The rectangular sections at the top of the collector are doors that are removed for bag and Venturi access.

Pulse jet collectors can be configured in a variety of ways. In some cases, the gas inlet must be located up high. Figure 7.7 is of a pulse jet collector designed with a high gas entry inlet. It is also equipped with a "walk-in" type CAP (the chamber located above the Venturis). Figure 7.8 shows a similar collector but equipped with a low-level gas inlet.

FIGURE 7.8

Low gas inlet pulse jet collector (Steelcraft Corp.).

Pulse jets can blast dirty, tacky product off the bag. If the particulate is moderately heavy or in clumps, it will drop into the hopper. If it is light or floats easily, it can get pulled right back onto the bag immediately after the short-duration cleaning pulse. Pulse jet self-cleaning cylindrical cartridge dust collectors use nominally 6-14-inch diameter x 26-inch-long pleated filters. Typical designs are shown in Figures 7.9 and 7.10. They were originally thought of as clean air filters because the filter design and cellulose media

FIGURE 7.9

Cartridge collector (Steelcraft Corp.).

type provided high cleaning efficiency. They were and still are used to clean ambient air or as final filters (after filters) following heavy-duty conventional fabric filter grade collectors. The pleats provided much more filter area than a round 4- or 6-inch diameter tubular bag. The filters less cages were short and easy to handle. The collector holding them could be compact. Filter service could be done in clean air outside the collector on the side of the unit. The problem was initially as cartridge units started to be sold as true front-line industrial collectors, the tight pleats would plug up due to heavy dust loading and blind the filters prematurely. To solve this problem, the perforated metal around the periphery of the filters was removed and pleat spacing was opened so dust could be blown out of the pleats easier and off the filter. Heavy-duty spun bond polyester media became popular. Filters were made with filter bag

FIGURE 7.10

Side-access cartridge collector (American Air Filter).

geometry allowing for replacement of round filter bags in other type collectors with pleated filters (more area) in the 4-6-inch diameter range. Currently many styles of self-cleaning pleated filters are used in industrial processing. They are compact, service easily, and can tolerate moderate loadings at high levels of cleaning efficiency. They use compressed air for cleaning energy like pulse jet baghouses. Although they are still not the best for heavy loadings and aggressive dusts, pleated filters continue to gain in the industrial marketplace. The fact is, nothing cleans easier than a smooth, round shape.

There are many types and versions of dust collectors within the various types. This is because there are myriad applications, and certain designs are best suited for certain applications. In selecting a collector for a given job, it is critical to understand the details of the process completely. It is also critical to understand how the collector works in detail so a match can be made.

Basically, the best dust collector for the job will require the least overall cleaning energy and cleaning cycles to perform. It will operate at low pressure differential over the filters, holding fan energy down, and will provide long efficient filter life and infrequent service.

This tells us that when the dirty air enters the collector, the dust/particu- late should take the shortest path to the hopper discharge and out. The filters should see only the lighter particulate/dusts that will build up a permeable filter cake to be cleaned off occasionally.

The prime considerations in collector design are inlet location and velocity and direction of dirty air flow inside the collector. For example, if the inlet is located below the filters, especially in a pyramid or conical hopper, all the air must go upward, directly impinging particulate into the filters. As the air/dust flows up between the filters, the air velocity (rising) increases, carrying the particulate up again and again into the filters. The dust has a hard time getting down past the inlet blast of air into and out of the hopper. On the other hand, if the dirty air inlet is located near the top of the filters, the dirty air flow must go downward directly toward the hopper or at worst horizontally onto the side of the filters. When filters need cleaning, the dust/ particulate cake simply drops off into a quiet hopper, reducing any potential for air pushing the dust upward back onto the filter medium.

Sizing fabric filters starts with an air-to-cloth ratio that field experience has shown will work on a certain application. The air (cubic feet per minute) to cloth (medium area) calculation gives us the face or impact velocity of dirty air as it hits the filter medium. Let us assume we have a ventilation process requiring 7200 acfm and the suggested ratio is 6/1; therefore, 7200/6 = 1200 ft cloth required in the dust collector (nominally), and 7200/1200 = 6 cfm/ft2 or 6 ft/min face velocity. As you can see, this provides us with a relative value for the volume and velocity of dirt and air flowing through the surface of the medium. The higher the gas velocity, the harder it is to push the dust off because you are pushing the dust back into the oncoming gas stream.

When using a compartmentalized off-line cleaning system, air-to-cloth ratio is a much less important factor as no process air is flowing into the filters. Cleaning off-line is quite easy at any air-to-cloth ratio.

Let us assume, again, that we are comparing two collectors, both processing 7200 cfm. The ratio being considered is nominally 6/1, meaning we need about 1200 ft2 of filter media. One collector, the tall unit, needs 60 filters/ cages at 6.2-inch diameter x 12 ft long to get approximately 1200 ft media. The filters are located on an 8-inch center grid pattern. The housing in the plan is 33.2 ft2; the filters in the plan, 11.76 ft2. The open area between the filters is 33.2 - 11.76 = 21.44. So, 7200 cfm/21.44 = 336 ft/min velocity. The other collector, the short one, needs 90 filters/cages x 8 ft long each. With all the other parameters and geometry, the same, the velocity between filters is only 233 ft/min. About 30% lower! The tall filter will be cheaper because it will have fewer filters, cleaning valves, and a smaller housing, but the fact is it will not perform as well as the shorter, fatter unit.

One way to determine acceptable can velocity as it relates to air-to-cloth ratio collector performance is to use an industry rule of thumb for maximum allowable rising velocity on particulate:

120 ft/min max for up to 10 lb. cu/ft product 240 ft/min max for up to 20 lb. cu/ft product 300 ft/min max for up to 30 lb. cu/ft product 360 ft/min max for up to 50 lb. cu/ft product 400 ft/min max for up to 70 lb. cu/ft product

Using lower velocity is always best. Products that float, like ultrafine light dust, bees' wings, feathers, and fiberglass fines, all need special consideration. Use collectors designed for that service. What we are doing here is comparing the terminal settling velocity of the dust particle in a relative sense to the velocity of the air between the filters. Four hundred mesh soft wood flour at 8 pcf is much harder to drop out in a hopper than 30 mesh silica sand at 75 pcf. Grain husks, paper trim, and fiber from buffing wheels act differently than 94 pcf Portland cement. Selecting or specifying a collector is really a matter of common sense and the experience of the user or manufacturer. In some cases, like dry S02 removal, we want a coating of soda bicarbonate on the filters; the same goes for pool lime on ultrafine dust or fume. In these applications, a substantial filter cake provides ultrafiltration. Using a modular setup with off-line cleaning is a good idea on these continuous bag coating applications.

For fabric filter collectors that use a precoat (such as for acid gas and/or mercury control), the air-to-cloth ratio for a reverse-air type collector is typically 2.5 to 1 or lower. If the collector is of pulse jet type, then the A/C ratio is about 3.5 to 1 or lower. The goal is often to keep the inlet flange to outlet flange pressure drop below 6-7 inches water column since the resistance of the collector impacts on the energy required to move the air through the system. In addition, with precoated type applications, the designer chooses the filter medium for efficient filter cake release. Various surface treatment techniques are used to tailor the filter cake retention to suit the application.

Air-to-cloth ratios are only guidelines. Many other factors affect performance. For example, the aspect ratio evaluates air-to-cloth ratio as it relates to dirty air velocities between filters in short or tall form collector. It is an especially important consideration because high velocity in low-inlet designs will not allow dust/particulate to drop down into the hopper.

Operating Suggestions

It should be obvious from the previous comments that to operate a fabric filter collector efficiently, it must first be sized correctly and then operated so that the collected dust (particulate) is removed properly. The mechanism to remove the particulate from the medium and the mechanism to remove the particulate from the hopper must be kept in good operating condition. If a shaker-type collector is used, the mechanical mechanism to shake the bags should be inspected and kept properly lubricated. If a reverse air-type unit is used, the reverse air isolation dampers and their actuators should be periodically inspected and maintained. These dampers and valves are critical to the reverse air's proper operation. If a pulse-type collector is used in cold climates, the compressed air supply should be conditioned or dried so that the fittings and valves do not freeze. The pulse timer (usually electronic) should be protected from voltage spikes so that its timing circuitry remains operable.

If the collector is used on a hot source containing acid gases (such as SO, and HC1) and periodically is shut down, the collector should be thoroughly insulated, and hopper heaters installed as needed. Some collectors utilize hot-air heating systems that recirculate air in the baghouse to uniformly distribute the heat. Failure to do so allows the baghouse environment to pass below the acid dewpoint, which causes localized corrosion and damage.

For pulse-type collectors, various Venturi and cage materials of construction (MOC) are available. These include coated Venturis, alloy wire cages, and so on. If the application is corrosive, attention should be paid to the MOC of the Venturis and cages. If the dust is explosive, special bags with grounding wires can be installed. Obviously, the grounding system should be inspected often to make certain that it is operating as intended.

For a hopper discharge problem in which the dust tends to bridge over the dust outlet, bin activators (shakers) or acoustic horns can often be used to break up such bridging. Usually, a continuous flow of dust out of the collector is better than an "accumulate and dump" type scenario.

On pulse type units, the pulse headers can often be removed from the top (clean air side), but space must be allowed for their removal. Some designs allow for the headers to be pulled out laterally. Again, one must plan for their removal.

If a bag breaks, you usually are in trouble. For that reason, various vendors offer broken bag detectors that scan the CAP for signs of particulate. If a broken bag is found, it is not uncommon to replace the row in which that bag was found as well as the adjacent rows. When one bag fails, it usually is a sign that others will follow.

To reduce bag injury upon installation, the bag tube sheet holes should be thoroughly deburred. New bags should be installed vertically (if that was their original orientation), not on an angle. This prevents the cage from chafing the medium.

On pulse-type units, the bag pulse frequency and duration should be carefully selected (most vendors have their required settings based upon experience). The pulse start sequence can often be initiated by a pressure switch so that a precoat of particulate can build up first. Every pulse in some small measure reduces the life of the bag, so pulsing should be done only as needed.

Shaker-type collectors often have media tensioning devices that require initial setup and checking. The collector manufacturer asks that these measures be followed to get the most use from the media. Unfortunately, these details are often overlooked.

On applications involving the temporary retention of a filter cake, over pulsing or over shaking the medium can shorten the life of the medium.

Fabric filter collectors provide excellent service when properly applied to the application and when they are operated as the designer intended.

 
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